1. Field of the Invention
This invention relates to electrical protective devices. More particularly, it relates to devices for protecting communications circuits against excessive voltage surges and excessive currents.
2. Description of the Prior Art
In telephone engineering, it is usual practice to provide protectors at central offices for each incoming line. These protectors, which may be termed units or modules, combine protection against excessive voltages resulting from lightning, for example, with protection against sneak currents. Sneak currents are not strong enough to do any damage if they flow briefly, but may generate enough heat to char conductor insulation and do other damage if allowed to persist. The sneak currents are produced by voltages of relatively low magnitude as compared to the excessive voltages mentioned hereinabove and usually result from accidental interference between telephone lines and adjacent power lines.
Protection of a telephone line against excessive voltage is usually provided by a so-called spark-gap protector which generally includes a pair of spaced carbon electrodes or a gaseous discharge device. One of the electrodes is usually connected to ground, and the other is usually connected to the incoming telephone line. Should a high voltage be impressed on the line, it will bridge the space or gap between the electrodes and cause current to flow to ground, thus bypassing sensitive equipment which is associated with the line.
The second type of protection is commonly provided by a device that is referred to as a heat coil. The heat coil includes a coil of small gauge, high resistance wire which is wound on a metal sleeve inside of which a contact pin is held in a predetermined position by a fusible bonding material such as solder, for example. Should excessive currents occur on the line and persist, sufficient heat will be generated by the coil of wire to melt the solder and release the pin. A spring is usually provided which urges the released pin into electrical contact with a source of ground potential to ground the line and protect sensitive line equipment.
A protector assembly of this general type is disclosed in U.S. Pat. No. 2,546,824, which was issued to P. P. Koliss on Mar. 27, 1951. A contact pin of a heat coil subassembly protrudes into a bore that extends through one of two carbon block electrodes of the spark-gap protector and is releasably held in a sleeve by a solder joint. It includes a pair of springs, one of which retains the elements of the assembly in abutting relation. When the pin is released by current build-up in the heat coil that melts the solder joint, the other spring urges the contact pin through the one carbon electrode into engagement with the other electrode which is connected to a source of ground potential.
Inasmuch as a ring conductor and a tip conductor are associated with each telephone station apparatus, each telephone line requires two protector assemblies. A telephone circuit protector unit, shown in J. B. Geyer et al., U.S. Pat. No. 3,573,695, which issued on Apr. 6, 1971, and which is incorporated by reference hereinto, includes two protector assemblies enclosed in a single insulative housing to save space, to protect the assemblies from dust, and to facilitate installation. Each protector assembly includes a spark-gap subassembly, having spaced carbon blocks, for excessive voltages, and a heat coil subassembly for excessive currents. The spark-gap and heat coil subassemblies are held in abutting-aligned relation by a single spring which is part of the normal transmission circuit. The spring also serves to propel a pin of the heat coil subassembly into engagement with a grounding circuit, which includes one of the carbon blocks, during the passage of excessive currents through the heat coil. In Geyer et al., the axis of each heat coil pin is aligned axially with the axis of its associated carbon blocks.
While units such as those described hereinabove have proved very useful in protecting telephone circuits from excessive voltages and currents, efforts have been made to introduce improvements. For example, to complete a fault current path to ground, the pin in the heat coil subassembly must be brought into contact with a carbon block in the spark-gap protector subassembly. This causes excessive heating of the spark-gap subassembly which becomes part of the fault path. Heat build-up in the carbon blocks of the spark-gap subassembly is commonplace because of their relatively high resistance. A further disadvantage is that the physical arrangement of the heat coil subassembly utilizes excessive space within the protector module. This, together with the extension of a contact pin through the voltage protection portions of the protector, has precluded the use of gaseous discharge devices in place of carbon blocks. Gaseous discharge devices are desirable because of their longer lives and because they afford better control of the breakdown voltage. Further, the need for recesses in the carbon blocks prevents heat shielding of these elements. Because the carbon blocks are provided with recesses, oftentimes, particles which become dislodged drop into and short the spark-gap. As a result, normal spark-gap-type operation may be precluded.
These last-mentioned problems have been overcome by a protector unit shown in U.S. Pat. No. 4,215,381, which issued on July 29, 1980, to R. F. Heisinger, and which is incorporated by reference hereinto. The unit includes a heat coil subassembly for sensing excessive currents and a voltage surge limiter assembly which is axially aligned with the heat coil subassembly for conducting excessive voltages through a grounding subassembly to ground.
In the Heisinger arrangement, gaseous discharge devices may be used inasmuch as the voltage protection portion of the protector is taken out of the fault circuit. When sufficient heat is transferred to the heat coil subassembly such as by a current fault, a fusible alloy melts to allow a spring to cause a heat coil flange to move and touch a laterally projecting tab of a ground terminal assembly. This creates an electrical path external to the voltage protector subassembly through to the ground terminal assembly. If a prolonged voltage surge occurs, there is an arcing over in the voltage surge limiter assembly, the fusible alloy is melted, and the spring moves the heat coil flange plate as before.
Although the Heisinger protector unit overcomes the problem of prior art arrangements, which precluded the use of gaseous discharge devices for voltage surge protection, it continues to use a spring as part of the normal and fault current circuits. Since the spring moves slidably, insulating sleeves are disposed about the spring to prevent shorting. At times, the presence of the spring in the talk circuit results in noise on the line.
A protector unit in which the spring is not in the talk circuit is disclosed in U.S. Pat. No. 4,168,515, which issued on Sept. 18, 1979, to B. W. Baumbach. That unit includes two heat coil subassemblies which are, together with the line pins, individually mounted on dielectric sub-bases. Both sub-bases are supported by a separate main base structure of the protector. The ends of the heat coil are welded to conductive plates to which the line pins are staked, the windings of the coil being directly in the line circuit. During over-current conditions, a fusible alloy is melted by the heat coil causing a conductive cup to move into engagement with a conductive plate that is connected to a line pin, thereby forming a direct metallic shunt to ground. In order, however, for the protector components to fit within a standard-sized housing, which is necessary for interchangeability with other protector units, a thin main base supports the two sub-bases. Such a thin base provides less support for the line pins. Furthermore, the local side stresses imparted on the holes within the main base may result in their permanent deformation and pin misalignment.
A relatively simple protector device having a minimum number of components, and in which there is no spring in the talk circuit, is the subject matter of U.S. patent application, Ser. No. 383,385, filed on even date herewith by J. L. Chapman, P. S. Nelson, and T. A. LaValle. In this protector assembly, current protection is effected by a heat coil subassembly which includes a sleeve disposed concentrically about an entension of the line pin through the supporting dielectric protector base. This sleeve is releasably secured to the line pin by a fusible bonding material and has convolutions of wire wrapped thereabout. One end of the resistance wire is welded to one end of the sleeve, which engages a voltage protection subassembly. The other end of the wire is welded to the central office pin mounted through the base.
In manufacturing protector assemblies of this type, the convolutions of wire cannot be wound around the heat coil sleeve prior to being disposed on the line pin. Since one end of the wire must be welded to the central office pin on the base, a free end of wire would have to be left on a prewound sleeve, which would be difficult to mechanically locate for welding to the pin. Furthermore, a nonfixed wire-wrapped sleeve would tend to unwind, leaving air gaps, which would change the heat transfer to the sleeve. Accordingly, mechanical assembly is best effected by affixing one end of the wire to the sleeve, winding the wire thereon, and affixing the other end of the wire to the central office pin. Difficulty arises, however, in automechanically implementing such procedure for manufacturing a protector unit which includes the standard two protector assemblies necessary for individually protecting the tip-and-ring conductors of a telephone circuit. Because of the close proximity of the two heat coil sleeves, the machine operations of welding and wire-wrapping would be extremely cumbersome.